Ink jet printing systems are known in which a print head defines one or more rows of orifices which receive an electrically conductive recording fluid from a pressurized fluid supply manifold and eject the fluid in rows of parallel streams. Printers using such print heads accomplish graphic reproduction by selectively charging and deflecting the drops in each of the streams and depositing at least some of the drops on a print receiving medium, while others of the drops strike a drop catcher device.
In ink jet printers of the continuous type, drops are selectively charged or left uncharged in response to the voltage on the charging electrodes at the time of drop break off. The formation of an uncharged print drop requires that the drop break off within the time interval of the zero voltage print pulse. A print error is produced if the drop fails to break off during the falling or rising edge transitions of the print pulse. To guard against this occurrence, it is necessary to adjust the phase of the print pulse relative to drop break off. As the drop break off phase with respect to the drive signal is very difficult to continuously monitor in a printer, the print pulse phase is set relative to the stimulation drive signal.
In known continuous ink jet printers, periodic rephasing procedures involving measurements of either drop charge or drop deflection are used. These procedures generally involve determining the desired print pulse phase with respect to the drive signal by monitoring the drop charge or deflection as the print pulse phase is stepped through the allowed range relative to the stimulation drive signal and detecting the desired result. The print pulse phase is then fixed relative to the stimulation drive signal. As the drop break off phase can drift relative to the stimulation drive signal, the printer must be periodically rephased. Depending on the printer, this rephasing period varies from one per document to one per hour, with a minimum of one each time the printer is started. Due to the small charge and the deflection of the drops, the phase test measurements have fairly low signal-to-noise levels, and can be plagued by ink mist build up, contaminating the detector. These periodic rephasing tests therefore add to the cost of the printer and can cause reliability problems.
Additional problems can be encountered with traveling wave stimulation, as disclosed in U.S. Pat. Nos. 4,999,644, and 4,972,201, due to the phase delay produced by the propagation time and attenuation of the flexure wave down the orifice plate. Furthermore, the phase of flexure wave can drift in response to temperature induced changes in the damping materials employed at the ends of the orifice plate. In piston type drop generators, such as is disclosed in U.S. Pat. No. 4,554,558, the resonating piston or crystal produces a pressure modulation at the back of the fluid cavity. This modulation produces a standing or resonating pressure wave in the fluid cavity. The phase of the pressure modulation at the orifices can vary relative to the modulation phase at the back of the cavity in response to temperature or concentration induced changes in ink sound velocity.
It is seen then that there is a need for an improved print pulse phase control which does not require either drop charging or drop deflection measuring means to determine the operating phase, or the need for periodic rephasing operations.